ecs-fundamentals-isystem-default

Your Unity DOTS ECS system allocates GC memory because managed references or a reachable [BurstDiscard] attribute in OnUpdate forces a silent fallback to non-Burst C#. Defaulting to ISystem prevents these managed allocations and preserves Burst compilation.

Use ISystem instead of SystemBase in Unity Entities 1.x for runtime hot-path ECS systems to reduce GC pressure and maintain Burst compatibility. Reserve SystemBase only for systems requiring unavoidable managed work.

Handle managed references in a Burst-ready Unity DOTS system by implementing a bridge-system pattern. Split managed-only setup into an adjacent SystemBase that produces unmanaged data for your ISystem hot path to consume.

The [BurstDiscard] silent-fallback trap in Unity ECS occurs when a managed call is reachable from OnUpdate, causing the system to compile but run in non-Burst C#. Enforce ISystem and isolate managed code to prevent this performance regression.

Refactor an ECS system with managed work into a Burst-compatible ISystem by extracting managed operations into an adjacent SystemBase bridge. This bridge processes managed setup and outputs unmanaged data for the ISystem hot path.

You cannot use SystemBase for performance-critical Unity DOTS systems without causing GC allocation because SystemBase is a managed class. ISystem is unmanaged and Burst-friendly, making it the required default for zero-GC hot paths.